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Acidity measurements in Me,SO solution have revealed that C6F5CHzCN is 5.9 pK, units more acidic than CsH5CHzCN due to the large field/inductive effects of the five fluorine atoms. Smaller CSx5 substituent differences (ApK,) were observed for parent acids where steric hindrance interfered with effective overlap with the p orbital of the carbanion, as for CH3C02Et (4.4 units), CH2(C02Et)CN (2.9 units), and fluorene (9-position; 3.2 units). The much larger ApK, (8.9) for p-(HC6F4)&H vs (C6F5)&H2 compared to (C6H5),CH vs (C6H5)&H2 (1.9 units) points to more effective carbanion stabilization in the tris(polyfluorophenyl)methanes, which is consistent with the appreciable acid weakening effects observed for the p-Me and p-Me0 groups in ( J J -M~C~F~)~C H and ( pMeOC6F4),CH. The 11.8 pK, unit increase in acidity for 1,2,3,4,5,6,7,8-octafluorofluorene, relative to fluorene, amounts to a 5.9 pK, unit acidifying effect for the four fluorine atoms in each ring, which matches the effect caused by five fluorine atoms in C6F5CHzCN.An impressive body of experimental evidence has accumulated that demonstrate significant stabilizing effects by fluoro and polyfluoroaryl substituents on carbanions. The evidence includes marked acceleration of the rates of carbanionic anomalous reaction chemi~t r y ,~,~ and enhanced hydrocarbon acidities, relative to analogous all-hydrogenIn our continuing studies on equilibrium acidities in Me2S0 solution,8 we have discussed the acidifying effect of replacing one or two of the hydrogen atoms in parent carbon acids such as CH,CN, CH,CO2Et, diphenylmethane, fluorene, and the like by phenyl group^.^ These studies have now been extended to some polyfluoroaryl substituents. A dramatic exaltation of ion-pair acidities in cyclohexylamine for (C6F5)CH2 and (C6F5),CH, relative to the parent (C6H5)&H2 and (C6H5),CH carbon acids, has been reported earlier: Le., replacement of each C6H5 group by c6F5 increased the acidity by 5-6 pK units. The present study confirms and extends these observations. Fluoro and Polyfluoroaryl Substituent Effects on Acetonitrile, Ethyl Acetate, and Ethyl Cyanoacetate. Examination of Table I shows that o-F, m-F, and p-F atoms change the acidity of C6H6CH2CN by 1.3,1.75, and -0.5 pK unit, respectively. The results indicate that the acid-weakening resonance effect overshadows the acidstrengthening fieldlinductive effect for p-F but not for o-F. The acidifying effect of the C6F6 moiety in C6F5CH2CN, relative to C6H6 in C6H5CH2CN, is 5.9 pK units, which is slightly greater than the sum of the o-F, m-F, and p-F effects (5.6 pK units).The acidifying effect brought about by substituting a C6F5 function for a hydrogen atom in CH3CN is 15.8 pK, units (21.6 kcallmol) as compared to 9.8 pK, units for C6H5. (This and subsequent ApK, comparisons are statistically corrected for the number of acidic hydrogen atoms present.) The difference of 5.9 pK, units represents the sum of the fieldlinductive effects of the five fluorine atoms. Substitution of two C6F5 functions for hydrogen atoms in CH3CN in...

The rate constants for reactions of a family of 19 carbanions, ArCHSO2Ph− (derived from benzyl phenyl sulfones) with n‐butyl chloride have been measured in Me2SO solution. A plot of log k vs. the pKa of the conjugate acids for 12 of these carbanions give a linear plot (R2 = 0.999) with a Brønsted coefficient of βNu = 0.402. Points for para electron‐withdrawing substituents, SPh, SOPh, SO2Ph. COPh, CN and NO2, deviate substantially from the plot. The deviations are attributed to the enhanced solvation of these remote substituents in the anion which leads to rate retardation. A curved Brønsted plot can be drawn through all the points, which would be consistent with the predictions of the Hammond–Leffler postulate (HLP) and the reactivity–selectivity postulate (RSP), but this interpretation is rejected. Instead, it is suggested that the apparent curvature in Brønsted plots for acid–base reactions – upon which HLP and RSP are based – is also caused by deviations due to solvent effects, donor atom effects in the bases, mechanistic changes and/or the failure to keep electronic and steric effects constant. A reactivity–selectivity plot for reactions of 9 ArCHSO2Ph− ions with n‐butyl bromide and n‐butyl chloride indicated constant selectivity. A similar plot for 4 carbanions derived from α‐methylbenzyl phenyl sulfones reacting with n‐butyl bromide and n‐butyl chloride also showed constant selectivity. Based on these results and an examination of the literature, it is concluded that there is no firm experimental basis for HLP and RSP.

The excellent linearity (R2 = 0.997) of a plot of pK, values for 17 m-and p-substituted benzyl phenyl sulfones, GC6H4CH2S02Ph, vs. those for the corresponding arylacetonitrile, GC6H4CH2CN, demonstrates that substituent solvation and substituent solvation assisted resonance (SSAR) effects for p-CN, p-COPh, and p-SPh are nearly identical in these two substrates. The PhS02 group in PhCH2S02Ph increases the BDE of the a-C-H bond by 2 kcalimol, relative to toluene. The a-C-H bonds in GC6H4CH2S02Ph sulfones are stabilized by 1-2 kcalimol by acceptor G's (m-CN, p-CN, rn-CF3, p-CF3), but weakened by 1 and 5 kcalimol, respectively, by donors (p-OMe and p-NMe2). The GC6H4CH2S02Ph+' radical cation with G = H has a pKHAA = -25. Its acidity is increased when G is an acceptor by as much as 9 to 10 kcal/mol (G = 3-CN, 3-CF3, 4-CF3, 4-N02). but is decreased when G is a donor by as much as 33 kcalimol (G = NMe2). When G = 4-SPh the radical cation is stabilized, relative to G = H, by a larger amount (25 kcal/mol) than when G = 4-OMe (18 kcalimol). Structural changes along the series PhCH2S02Ph, 2-naphthyI-CH2SO2Ph, 9-anthrylCH2S02Ph cause negligible changes in the acidities of these acids, but sizable decreases in the acidities of the corresponding radical cations. Introduction of a phenylsulfonyl group into the methyl group of 9-methylanthracene or the 9-position of fluorene or xanthene increases the BDEs by 3, 2, and 7 kcalimol, respectively. These effects of PhS02 groups are compared and contrasted with those of CN groups. conformational analysis, is about 2.5 kcal/mol compared to about 0.2 kcal/mol for the linear cyano f~n c t i o n .~The appreciable ability of CN to stabilize an adjacent carbon centered radical and the poor ability of RSOz functions to do so was revealed long ago in copolymerization ~t u d i e s .~ Recent rate studies of the thermolysis of Me2C(G)N=NC( G)Me2 azo compounds also point to a large disparity in the ability of these two functions to stabilize radicals. Thus the rate for G = CN is 2-9 x lo5 times greater than for G = Me = (l-O), as compared to only 9.2 for G = CH3S0z.5 The apparent stabilizing effect of a p-CN group on a benzyl radical is also far greater than that of ap-MeSOz group, as judged by e.s.r. hyperfine coupling constants (a*, = 0.040 for p-CN vs. 0.005 for p-MeSOz).6 E.s.r. studies of cyanomethyl radicals indicate substantial spin dejocalization ability,7 whereas a-sulfonyl groups have 'no capacity for spin delocalization .'* By combining data for equilibrium acidities in Me2S0 solution (pKHA) with (a) the oxidation potentials of their conjugate bases, EOx(A-), and (b) a summation of four constants dictated by a thermodynamic cycle we have devised a method for estimating homolytic bond dissociation energies (BDEs) for weak acids (HA) in Me2S0 solution (equation (l)).9 In the preceding paper" an estimate using equation (1) has shown